1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a pixel circuit of an organic electroluminescent display device and a method of driving the same.
2. Description of the Related Art
An organic electroluminescent display device (or organic light emitting diode display device) is a flat panel display device that electrically excites an organic material (e.g., phosphorous organic compounds) to emit light. In an active matrix organic electroluminescent display device, a capacitor stores a voltage for representing a predetermined gray level, and the stored voltage is applied to a pixel for the entire duration of a frame. Based on the type of signal applied for storing the voltage in the capacitor, the active matrix organic electroluminescent display device can be classified into an active matrix organic electroluminescent display device using a voltage programming method or an active matrix organic electroluminescent display device using a current programming method.
Unlike a liquid crystal display (LCD) using voltage driven liquid crystal, the organic electroluminescent display device using the current programming method employs a current driven organic light emitting diode (OLED: also referred to as “an organic EL diode”). Therefore, the organic electroluminescent display device emits light at a luminance controlled by a driving current. Further, the organic electroluminescent display device includes a pixel circuit to generate the driving current.
FIG. 1 is a circuit diagram of a pixel circuit of a conventional organic electroluminescent display device, and FIG. 2 is a timing diagram for driving the pixel circuit of FIG. 1.
Referring to FIG. 1, the conventional pixel circuit includes first, second, third, and fourth transistors M1, M2, M3 and M4, first and second capacitors C1 and C2, and an organic EL diode OLED.
The first transistor M1 controls a current flowing to a drain thereof according to a voltage applied between a gate and a source thereof. The second transistor M2 applies a data voltage to the first capacitor C1 in response to a selection signal supplied from a scan line Sn.
The third transistor M3 connects the first transistor M1 to function as a diode in response to a selection signal supplied from a scan line AZn. The fourth transistor M4 transmits a driving current from the first transistor M1 to the organic EL diode OLED in response to a selection signal from a scan line AZBn.
The first capacitor C1 is connected between the gate of the first transistor M1 and a drain of the second transistor M2, and a second capacitor C2 is connected between the gate and the source of the first transistor M1.
Hereinafter, an operation of the conventional pixel circuit of FIG. 1 will be described in more detail with reference to FIG. 2.
First, when the third transistor M3 is turned on by the selection signal from the scan line AZn, the first transistor M1 is diode-connected, so that a voltage VDD−|Vth| is at a node N at which the first capacitor C1 and the second capacitor C2 are connected.
Then, when the third transistor M3 is turned off and a data voltage Vdata is applied, the voltage at the node N changes by as much as a variation ΔV=VDD−Vdata in the data voltage applied in the first capacitor C1. Therefore, the voltage at the node N changes into VDD−|Vth|−ΔV.
Then, when the selection signal from the scan line AZBn is applied, the fourth transistor M4 is turned on so that a driving current flows to the organic EL diode OLED.
The driving current IOLED flowing to the organic EL diode OLED can be obtained by the following Equation 1:IOLED=k(Vgs−|Vth|)2=k(VDD−VDD+|Vth|+VDD−Vdata−|Vth|)2=k(VDD−Vdata)2  [Equation 1]Here, VDD is a power supply voltage, Vth is a threshold voltage of the first transistor M1, and Vdata is the data voltage.
As shown in Equation 1, the above described conventional pixel circuit includes the first and second capacitors C1 and C2, and the third and fourth transistors M3 and M4, to compensate for a difference in threshold voltages of first transistors M1.
However, because the conventional pixel circuit needs three different scan lines Sn, AZn, and AZBn, the pixel circuit and the driving circuit are complicated and an aperture ratio of a light emitting display device including the pixel circuit is low.
Further, while one pixel is selected, the data is programmed in the conventional pixel after the difference in the threshold voltage is compensated for. Thus, a charging problem (or delay) makes it difficult to apply the conventional pixel circuit to a high-resolution panel.
Further, in the conventional pixel circuit, the driving current IOLED is controlled by adjusting the power supply voltage VDD and the data voltage Vdata, but a pixel close to the power supply voltage VDD and a pixel far from the power supply voltage VDD have different voltage drops (IR-drops) of the power supply voltage VDD. Therefore, even though substantially the same data voltage Vdata may be applied to the pixels, the luminance may still be non-uniform.
Also, the power supply voltage VDD for driving the conventional pixel circuit should be smaller than or equal to a maximum gray level voltage of the data voltage Vdata. In general, the data voltage Vdata has the maximum gray level voltage (or a black data voltage) of about 5V, so that the power supply voltage VDD should not be higher than 5V. Therefore, a reference voltage VSS needs to have a negative voltage (about −6V) to maintain a voltage difference of 11V between the power supply voltage VDD and the reference voltage VSS. This voltage difference reduces the efficiency of a DC-DC converter supplying the power supply voltage VDD and the reference voltage VSS.
As such, it may be desirable to design a new pixel circuit to address the foregoing problems.